What is the difference between `type()` `__class__` and `__bases__` in Python?

Question

I tried this in Python's REPL:

>>> class Foo:
...  def f():{}
... 
>>> 
>>> type(Foo)
<type 'classobj'>
>>> Foo.__bases__
()
>>> type(type(Foo))
<type 'type'>
>>> type(Foo).__bases__
(<type 'object'>,)

However, I still can't figure out what "data type" means in OOP exactly.

In Python, I know that an instance can get its class by .__class__ and a class can get its parent class by .__bases__. This seems easy to understand.

But what does the the type of a "Class", or TypeObject, mean? And what does the type of a Type Object mean? What does the __bases__ of a Type Object mean? What is the difference between type and class in Python?

This looks a bit confusing to me.. Does anyone have ideas about this?

Answer 1

The key to understanding is recognizing that everything in python is an object. Also, technically object isn't a keyword, it's a global built-in.

This answer is perhaps the best explanation I've ever seen on the topic. I highly recommend reading the entire post. It talks a lot about metaclasses, and type is very much tied to metaclasses.

Essentially, the type of a Class is a MetaClass. And in the same way that all new-style classes in python inherit from object, all metaclasses inherit from type. It's a bit confusing because the type global in python is used for two different purposes. It's a metaclass and a function that returns the type of an object.

A very simple way to think of it is that metaclasses create classes, in the same way that Classes create instances.